Bodies made of particulate inorganic materials such as ceramic materials, activated carbon and molecular sieves, are used in a wide variety of applications such as catalysts, supports for catalysts and adsorption media. One of the most common methods of manufacturing these bodies is by forming the body from a plasticized mixture. The plasticized mixture typically consists of a mixture of the respective inorganic material, usually in particulate form such as a powder, with binders, etc., carried in a suitable vehicle. The vehicle is typically a solvent such as water, organic solvent or mixtures thereof. The bodies are shaped using various methods dried and often heat-treated using such methods as calcination.
A particularly important class of structured bodies is those comprising molecular sieve like materials. Molecular sieve materials for use in catalytic and other applications are, for various reasons, rarely utilized in their as synthesized form. Usually such molecular sieve materials are formulated into compositions with other materials such as inorganic or organic binders and other co-active components to enable the molecular sieve to be formed into a support structure or catalyst structure. Typically this catalyst composition is formed into three-dimensional structures such as, for example, extruded pellets or more complicated structures such as extruded honeycombs. Various high surface area oxides such as alumina, titania, spinel, zirconia, silica, and the like, are known in the art as being extrudable into high strength supports, such as honeycomb structures used for filters or used for catalyst supports and for being extrudable as catalyst pellets. Some of these materials such as the amorphous silicas are relatively difficult to extrude. Molecular sieve materials such as, for example, zeolites or mesoporous crystalline silica materials have not been readily available in extruded form or have been extruded with some difficulties and limitations.
Molecular sieve materials require careful formulation with binders and other additives to form compositions that are readily formable, for example, by extrusion, to prepare molecular sieve structures. However, despite this care, the final molecular sieve structures may exhibit certain deficiencies and problems as a consequence of the composition and/or the forming method used. These problems are often due to the need to use high levels of organic or inorganic binder in combination with solvents to extrude the molecular sieve structures. The resultant structures, after calcination, contain binder residue or damage due to the manufacturing process. In addition the high levels of binder result in extruded structures that contain reduced levels of active molecular sieve material. A further difficulty is in providing extruded molecular sieve structures that have sufficient compressive strength. Acceptable compressive strength is important to ensure that the molecular sieve structures remain intact and retains their activity during use. If these structures are of insufficient strength, they may deteriorate quicker in use and require replacement at more frequent intervals; this may have significant detrimental effects on the economics of the processes in which they are used. A further requirement with extruded molecular sieve structures is that they have sufficient green strength on extrusion to enable ease of handling. A further problem is that the final extruded molecular sieve structure may have reduced or modified activity, which is due to the unavoidable presence of co-extruded binders that affect the effective catalytic activity during use. One particular class of molecular sieve materials that suffers from this problem is the high Si/Al ratio nanoporous materials that are normally extruded with caustic/colloidal silica binders to produce high silica content extrudates, such as those described in U.S. Pat. No. 4,582,815. In these extrudates the binder causes damage to the inherent structure of the molecular sieve. One of these deficits is the significant loss of surface area of the molecular sieve crystal or mesoporous material under these high pH conditions. The high surface area mesoporous crystalline aluminosilicates and silicates designated as M41S, such as MCM-41 and MCM-48, have been particularly difficult to extrude as molecular sieve structures with high strengths using conventional extrusion techniques.
Further to the above-identified problems, there are also difficulties associated with solvent-based systems. There are OSHA concerns (toxicity and flammability) due to the presence of strong organic solvent vapor during mixing and extrusion. There are also difficulties associated with drying when solvent is used in the manufacture of the extruded material. Under normal drying in an electrically heated oven, volatile solvents tend to cause blistering and cracking of such molecular sieve structures due to a rapid forming of dry skin over the substrates. To minimize the occurrence of such problems, the structures are generally dried at very slow rates in a ventilating hood over a period of several days. The slow drying is due to the tendency of such structures to rapidly form a dry skin over the substrate from the volatile substrate, which also causes blistering and cracking when such structures are subjected to drying. As a result, molecular sieve structures formed from solvent-containing batches require slow drying in a ventilating hood, typically over a period of days to minimize the occurrence of blisters and cracks.
In U.S. Pat. No. 5,633,217, zeolite honeycomb extrudates are manufactured using silicone resins as a permanent binder. As disclosed in this reference, except for silica, precursors of the permanent binder may be used in the form of a dispersion, suspension, or solution in a liquid diluent. When the precursor for the permanent binder is a silicone resin, the resin is dissolved in an organic solvent or a mixed organic solvent system of alcohol and water. The silicone resin may be mixed directly with porous oxide powders, in which case a solvent is used during the plasticizing step to dissolve the resin. Alternatively, the silicone resin may be pre-dissolved in an appropriate organic solvent such as methyl alcohol, ethyl alcohol and isopropyl alcohol. Whether the silicone resin is pre-dissolved in a solvent or mixed directly with the porous oxide powders, followed by the addition of solvent, the resins may be milled. In this reference, it is taught that the use of organic solvent is essential in order to achieve an extruded monolith with acceptable flexural strength. In Example 2 (Comparative) of U.S. Pat. No. 5,633,217, Dow Corning 6-2230 silicone resin was used in the form of flakes; these are known to be of particle size in excess of 1 mm. In this Example, the resin was used without organic solvent to form an extruded rod, which had a reported flexural strength of less than 100 psi; this was described as a low flexural strength.
U.S. Pat. No. 5,492,883 offers the possibility of reducing the levels of organic solvent used in the manufacture of extruded molecular sieve structures. In this reference, zeolite extruded structures are prepared using an aqueous emulsion of silicone resin. The emulsions typically contain silicone particles of the order of 700 nm particle size and also contain 60 wt. % resin solids. However, the emulsions also contain 0.5 to 1 lb/gal of aromatic solvents. This indicated level of solvent is described as low in this reference. However, the present environmental standards dictate that this level of organic solvent is, in many instances, an unacceptably high level.
Acceptable green strength is important in order to enable ease of handling of extrudates during their manufacture and, in addition, to ensure that the extrudate maintains its shape and does not collapse or deform under its own weight during the manufacturing process. Extrudates prepared using organic solvent-based compositions are known to have poor green strength and to be easily deformed after extrusion. Therefore, there is a need for alternative and more flexible methods for the manufacture of structured bodies and, in particular, formed molecular sieve structures without the use of organic solvents.